One of the most disappointing things about America's Founders and Framers is that they were so... well, imperfect. We want our heroes to be the stuff of myth, dispensing their Zeusian wisdom on high from Mt. Olympus, so it's hard to get past the fact that Jefferson - he of "inalienable rights" and "all men are created equal" - Madison, and others of their peers and colleagues, were slaveowners.
I lurk on a listserv for constitutional law and political science profs, and recently there've been some illuminating exchanges on this point; in particular, comparing Thomas Jefferson and Alexander Hamilton.
A number are highly critical of Jefferson for his hypocricy on the slavery question. Professor Sean Wilson from Penn State comments, for example, "I think [Jefferson's] opposition to the Missouri Compromise later in life (wanting slavery to expand everywhere), his non-opposition to voluntary emancipation in Virginia, and his opposition to the slave trade can be further understood in terms of his racism. Virginia had an excess supply of slaves, so all of these efforts would encourage fewer Americans of African descent to live in Virginia. He talked about the need to have "diffusion" before slavery might be outlawed (which is code for not having too many people around you)...."
Professor Paul Finkelman of Albany Law School says that when he began his extensive work on Jefferson, he "expected to take [a favorable] position.... I had read books about Jefferson and assumed he was a moderately antislavery man, caught up in his world. I then read almost everything Jefferson has ever written on slavery and came to a very different conclusion. I have looked very carefully at his life and his treatment of his slaves. It is not a very pretty picture. His statements are very dishonest."
"[For example,] in Notes on the State of Virignia he makes it clear that they are not his 'moral equal.' He argues that they do not love like white people, they have not skill, they are in effect genetically inferior to whites. He compares them to Roman slaves, noting the many successes of Roman Slaves, and then goes on to note that Roman slaves were "white." In the Notes, written in 1783, he seems pretty certain that blacks are naturally inferior to whites in all ways. Other southerners argue the same thing. They also are already arguing that the Bible supports slavery in general and black slavery in particular.... Indeed, Jefferson's Declaration forces Jefferson and others to assert and develop a "scientific" racism to support slavery since otherwise the logic of the Declaration would be emancipation. Jefferson opposed emancipation his whole life, even telling friends not to free their slaves (they were "pests" "on society" he argued)."
Hamilton, by contrast, comes off much more favorably. Professor Matthew Holden from Virginia comments, "Hamilton supported the proposal of Laurens of South Carolina to free African slaves and arm them to fight the British, [stating,] 'The contempt we have been taught to entertain for the blacks makes us fancy many things that are founded neither in reason nor experience, and unwillingness to part with property of so valuable a kind will furnish a thousand arguments to show the impracticability or pernicious tendency of a scheme which requires such a sacrifice.'"
Professor Wilson adds, "(a) Hamilton saw the evils of slavery in St. Croy [sic] and the west indies fairly early in life; (b) Hamilton knew that a single-cash crop economy was an inferior economic model that was actually, in the long term, retarding the South's development into an industrial and manufacturing economy; (c) Hamilton appears to have been against both slavery and discrimination; (d) Hamilton never favored "repatriation;" and (e) both he and his wife were committed abolitionists throughout their own lives."
In comparing the two men, Finkelman concludes, "there is a huge difference between someone who "condemns" slavery -- that is says it might be wrong in the abstract, or that it is bad policy, or that it harms white people (that are Jefferson's points) and someone who actually does something about it in either is professional, public, or personal life. A number of men of the revolutionary era freed their own slaves either in their lifetime or at their death. Others, like Judge St. George Tucker, proposed ways of ending slavery. Jefferson did none of this."
All of that said, Professor Michael Curtis from Wake Forest suggests we should acknowledge the individual failings but look also at the larger contributions: "The 'poetic exaggeration' of the Declaration was a force for good--whatever the failings of the man who wrote it and the others who endorsed it. It helped to expand suffrage to the poor, to support freedom for the slave, to expand suffrage to women, to support civil rights for blacks and others, and to further equal rights for women. You can see its influence, I think, in section 1 of the 14th Amendment. Not a bad list of contributions for a 'poetic exaggeration.' It was, as Lincoln saw, an ideal--never fully attained but something to strive for. So Lincoln denied that the ideals of the Declaration could or should be degraded by the fact many of the framers did not live up to its ideals or to the practices they engaged in in 1776."